As a method for welding a work-piece configured of two members, friction stir welding is known. Friction stir welding is a welding method in which a tool is rotated in a state where an abutment portion of the work-piece is pressurized by a shoulder surface of the tool and in which the work-piece is welded using friction heat generated on the surface of the work-piece.
Meanwhile, in the friction stir welding, a tool referred to as a bobbin tool may be used. In the bobbin tool, a front side shoulder having one shoulder surface is disposed on a front surface side of the work-piece, a rear side shoulder having the other shoulder surface opposing the one shoulder surface is disposed on a rear surface side of the work-piece, and the two shoulders are connected to each other by a probe penetrating the work-piece. In this way, the front surface and the rear surface of the work-piece are interposed between the shoulder surfaces of the two shoulders and pressurized, frictional heat is generated, the work-piece is stirred by the probe, and thus, the friction stir welding of the work-piece is performed.
Here, in the work-piece, non-uniformity of plate thickness due to distortion or manufacturing errors may occur. Particularly, in an outer plate or the like which is used in a vehicle of a track transportation system referred to as an automated people mover (APM) or a train, a long hollow panel is used. Moreover, in the long member, significant change in the thickness in the length direction occurs. In addition, for example, when the plate thickness in the weld direction is thin, welding pressure from the shoulder surfaces to the work-piece decreases, and there is a concern that sufficient welding quality may not be obtained.
PTL 1 discloses a friction stir welding mechanism which controls a gap between shoulder surfaces by allowing a lower shoulder (rear side shoulder) to move close to and move away from an upper shoulder (front side shoulder) using hydraulic pressure according to a change in a plate thickness of a work-piece, and can adjust welding pressure with respect to the work-piece.